| A simple-command is a list of variable assignments (see Variable Assignments below) or a sequence of blank separated words which may be preceded by a list of variable assignments (see Environment below). The first word specifies the name of the command to be executed. Except as specified below, the remaining words are passed as arguments to the invoked command. The command name is passed as argument 0 (see exec(2)). The value of a simple-command is its exit status; 0-255 if it terminates normally; 256+signum if it terminates abnormally (the name of the signal corresponding to the exit status can be obtained via the -l option of the kill built-in utility). A pipeline is a sequence of one or more commands separated by |. The standard output of each command but the last is connected by a pipe(2) to the standard input of the next command. Each command, except possibly the last, is run as a separate process; the shell waits for the last command to terminate. The exit status of a pipeline is the exit status of the last command. Each pipeline can be preceded by the reserved word ! which causes the exit status of the pipeline to become 0 if the exit status of the last command is non-zero, and 1 if the exit status of the last command is 0. A list is a sequence of one or more pipelines separated by ;, &, |&, &&, or | |, and optionally terminated by ;, &, or |&. Of these five symbols,;, &, and |& have equal precedence, which is lower than that of && and | |. The symbols && and | | also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (i.e., the shell does not wait for that pipeline to finish). The symbol |& causes asynchronous execution of the preceding pipeline with a two-way pipe established to the parent shell; the standard input and output of the spawned pipeline can be written to and read from by the parent shell by applying the redirection operators <& and >& with arg p to commands and by using -p option of the built-in commands read and print described later. The symbol && (| |) causes the list following it to be executed only if the preceding pipeline returns a zero (non-zero) value. One or more new-lines may appear in a list instead of a semicolon, to delimit a command. A command is either a simple-command or one of the following. Unless otherwise stated, the value returned by a command is that of the last simple-command executed in the command. for identifier [ in word...] ; do list; done | Each time a for command is executed, identifier is set to the next word taken from the in word list. If in word... is omitted, then the for command executes the do list once for each positional parameter that is set (see Parameter Substitution below). Execution ends when there are no more words in the list. | for (([expr1] ; [expr2] ; [expr3]));do list;done | The arithmetic expression expr1 is evaluated first (see Arithmetic Evaluation below). The arithmetic expression expr2 is repeatedly evaluated until it evaluates to zero and when non-zero, list is executed and the arithmetic expression expr3 evaluated. If any expression is omitted, then it behaves as if it evaluated to 1. | | select identifier [ in word...] ; do list; done | A select command prints on standard error (file descriptor 2), the set of words, each preceded by a number. If in word... is omitted, then the positional parameters are used instead (see Parameter Substitution below). The PS3 prompt is printed and a line is read from the standard input. If this line consists of the number of one of the listed words, then the value of the parameter identifier is set to the word corresponding to this number. If this line is empty the selection list is printed again. Otherwise the value of the parameter identifier is set to null. The contents of the line read from standard input is saved in the parameter REPLY. The list is executed for each selection until a break or end-of-file is encountered. | case word in [ [(]pattern [ | pattern] ...) list;; ... esac | A case command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for file-name generation (see File Name Generation below). | if list; then list elif list; then list... ; else list; fi | The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If no else list or then list is executed, then the if command returns a zero exit status. | | while list; do list; done until list; do list; done | A while command repeatedly executes the while list and, if the exit status of the last command in the list is zero, executes the do list; otherwise the loop terminates. If no commands in the do list are executed, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test. | | (list) | Execute list in a separate environment. Note, that if two adjacent open parentheses are needed for nesting, a space must be inserted to avoid arithmetic evaluation as described below. | | {list;} | list is simply executed. Note that unlike the metacharacters (and), {and} are reserved words and must at the beginning of a line or after a ; in order to be recognized. | [[expression]] | Evaluates expression and returns a zero exit status when expression is true. See Conditional Expressions later in this appendix, for a description of expression. | function identifier {list;} identifier () {list;} | Define a function which is referenced by identifier. The body of the function is the list of commands between {and}. (See Functions later in this appendix). | time pipeline | The pipeline is executed and the elapsed time as well as the user and system time are printed on standard error. |
The following reserved words are only recognized as the first word of a command and when not quoted:
if then else elif fi case esac for while until do done {}
function select time [[ ]] !
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